A monopole antenna is a resonant antenna, with its optimal length being one quarter of the wavelength of the signal being received or transmitted by the antenna. The antenna impedance for a monopole is a function of the length of the monopole in terms of wavelengths. A quarter wavelength monopole antenna is about 36 ohms, and is purely resistive.
When the antenna is much shorter than a quarter wavelength of the signal being received or transmitted by the antenna, the antenna's impedance is mostly capacitive, with very little resistance. This situation generally occurs at the lower frequencies of the antenna's operating range (where the wavelength of the signal being received or transmitted is at the high end of its range), particularly in applications where the antenna structure must operate over a wide frequency band. In narrow band applications, an inductor or coil is used to negate the capacitive reactance. For wide band applications, however, this is problematic, as it becomes difficult to match impedance in order to couple sufficient power into the RF circuitry for good signal to noise ratio (SNR).
One conventional technique here is to use two antennas of scaled dimensions connected through a diplexer, where one antenna is configured to cover the lower portion of the frequency band, and the other antenna is configured to cover the upper portion of the frequency band. However, the lower frequency portion of the antenna must be of sufficient length to operate. In applications where physical space is limited, the antenna length required to provide sufficient gain may be too large.
What is needed, therefore, is a miniature broadband monopole antenna configuration.